1. Field of the Invention
The present invention relates to a sub-rate multimedia data transmission control system for transmitting information such as a motion video, a voice, and data at a sub-rate.
2. Description of the Prior Art
FIG. 8 is a diagram schematically showing a configuration example of a transmitter according to the conventional sub-rate multi-media data transmission system described, for example, in the CCITT Draft Recommendations H.130 Part 3 in which the configuration includes a motion video encoder 1, a transmission buffer 2 for interfacing or matching the code transmission speed with the encoded output delivered from the .[.moving.]. picture encoder 1 of which the amount of the encode data is not uniform, a video image frame data 3 read from the transmission buffer memory 2 at a code transmission speed, an error correction encoder 4 for effecting a (255, 239, 5) BCH encoding operation on the video image frame data 3, an error correction frame data 5 outputted from the error correction encoder 4, a 16-phase interleaver 6 for achieving a 16-phase interleaving operation on the error correction frame data 5, video image data 7 delivered from the 16-phase interleaver 6, a voice encoder 8, voice data 9 transmitted from the voice encoder 8 at a speed of 64 kbps, digital data 10 having various speeds, an external data multiplexing section 11 for multiplexing the digital data 10 with a speed conversion so as to transmit the data at a speed of 64 kbps, external data 12 sent from the external data multiplexing section 11 at a speed of 64 kbps, a control section 13 between apparatuses which effects a communication control with communicating apparatuses, control data 14 outputted from the control section 13 at a speed of 32 kbps, a multiplexed control signal 15 delivered from the control section 13, a multiplexing section 16 for multiplexing the video data 7, the voice data 9, the external data 12, and the control data 14 so as to send these data at a speed of 1.544 Mbps, and a transmission frame 17 sent from the multiplexing section 16 at a speed of 1.544 Mbps.
FIG. 9 is a schematic diagram illustrating a transmission frame configuration of the transmission apparatus of FIG. 8 according to the conventional sub-rate multi-media data transmission control system. The configuration includes transmission data 17 at a speed of 1.544 Mbps in which a transmission frame comprises 193 bits, a transmission frame bit F.sub.r 20 assigned to each transmission frame of the transmission data 17 and the use of which is repeated at an interval of 24 transmission frames, an odd-numbered frame 21 obtained by classifying the transmission data 17 into odd-numbered frames and even-numbered frames respectively, and similarly an even-numbered frame 22 thereof.
FIG. 10 is a schematic diagram showing a process in which the motion video data 3 is encoded for a transmission path in the transmission apparatus of FIG. 8 according to the conventional sub-rate multi-media data transmission control system. The configuration comprises a unique code word 30 indicating the top of a video image frame, encoded data of the first block line 31 located at the uppermost end of a screen when a video frame is structured in the screen in which a block line includes several lines in the horizontal direction and the block line is assigned as the minimum encoding unit, encoded data of the second block line 32 formed in the similar fashion, encoded data of the n-th block line (n is an integer at least equal to one) 33 similarly located at the lowermost end of the screen, 256-bit error correction frame data 5 which is obtained by effecting the (255,239, 5) BCH encoding on a 239-bit unit data beginning from an arbitrary position of the video image data and thereafter by adding an error correction frame bit 34, an error correction frame bit S which identifies a division point of the 16-phase interleave and a division point of the error correction frame and of which the utilization is repeated at an interval of 16 error correction frames, an information bit 35 of the 239-bit (255, 239, 5) BCH code obtained by dividing the image data 3 at an arbitrary position, a 16-bit error correction code Ecc 36 added to the information bit 35, transmission video image data 7 attained by effecting a 16-phase interleaving on the error correction frame 5, and data 38 obtained by achieving the 16-phase interleaving on the error correction frame 5 excepting the error correction frame bit S.
Next, the operation of the configuration above will be described. The encoded output resulting from the encoding operation of the motion video encoder 1 is temporarily stored in the transmission buffer memory 2 and thereafter is read therefrom at a code transmission speed, thereby matching the encode speed with the code transmission speed. The video image data 3 read from the transmission buffer 2 is inputted to the error correction encoder 4, which effects the (255, 239, 5) BCH encoding on the data to attain the error correction frame data 5. Next, the video data 7 having undergone the 16-phase interleave operation in the 16-phase interleaver 6 is sent to the multiplexer 16 so as to be multiplexed into transmission data at a speed of 1.544 Mbps. On the other hand, the voice data 9 encoded by the voice encoder 8 is fed to the multiplexing section 16 at a speed of 64 kbps. The digital data 10 at various speeds, for example, 1200 bps and 2400 bps .[.data is.]. .Iadd.is data .Iaddend.multiplexed by the external multiplexing section 11 according to the procedure described in, for example, the CCITT Recommendations X.50 into the external data 12 at a speed of 64 kbps and is then delivered to the multiplexer 16. In the controller 13 between apparatuses, information necessary to be sent to a communicating apparatus for the communication control with the communicating apparatus is delivered as control data to the multiplexer 16 at a speed of 32 kbps.
In the multiplexing section 16, the video data 7, the voice data 9, the external data 12, and the control data are multiplexed into a transmission frame 17 at a speed of 1.544 Mbps according to the predetermined frame configuration indicated by the multiplex control signal 15 sent from the controller 13 between apparatuses, thereby transmitting the multiplexed data to a transmission line.
Next, referring to FIG. 9, description will be given of the transmission frame configuration. According to the CCITT Draft Recommendations H.130 Part 3, the transmission is accomplished at a transmission rate of a so-called Primary Group (1.544 Mpbs) and a transmission frame 17 includes 193 bits; consequently, the frame repeat cycle is obtained as 1.544 Mbps/193 bits=8 kHz. Assume that bit 0 of the transmission frame 17 is a frame bit F.sub.r 20 and that the remaining 192 bits ranging from bit 1 to bit 193 are assigned to an information channel. The utilization of the frame bit F.sub.r 20 is repeated for each 24 frames according to the CCITT .Iadd.Data. .Iaddend.Recommendations G.704 and information such as the frame synchronization, the multiframe synchronization, the data link, and the CRC-6 are subjected to a time-sharing operation. The information bits of a transmission frame 17 are subdivided into items .[.TS1-TS.gtoreq..]. .Iadd.TS1-TS24 .Iaddend.each comprising 8 bits, and the multi-media data is multiplexed according to this unit.
First, the 24 multiframes are classified into odd-numbered frames 21 and even-numbered frames 22, TS1 and TS16 are respectively assigned to the voice data 9 and the external data 12, and TS2 of the odd-numbered frame 21 is assigned to the control data 14. The TS other than those above are assigned to the video image data 7. As a result, the multiplexing rate of each data becomes as follows.
Frame bit F.sub.r 20: 1 bit.times.8 KHz=8 Kbps PA1 Voice data 9: 8 bits.times.8 KHz=64 Kbps PA1 External data 12: 8 bits.times.8 KHz=64 Kbps PA1 Control data 14: 8 bits.times.8 .[.KHz 1/2.]. .Iadd.KHz.times.1/2.Iaddend.=32 Kbps PA1 Video data 7: 1.376 Mbps
In general, the bit synchronization and the 8-bit octet synchronization must be retained for the 64 Kbps voice data. In this case, however, the octet synchronization is established based on a fact that the data is multiplexed in the transmission frame 17 in a unit of eight bits.
For the external data 12, a frame pattern conforming to, for example, the CCITT Recommendations X.50 is inserted into the external data channel TS16 thereof so as to establish the frame synchronization independent of the synchronization of the transmission frame 17.
For the motion video data 7, since the required .[.bits.]. .Iadd.bit .Iaddend.error rate thereof is critical in general as compared with the error rate of the ordinary transmission path, the transmission path error countermeasurement is effected through the (255, 239, 5) BCH encoding and the 16-phase interleaving operation. Consequently, the start bit of the 16-phase interleaving and the start bit of the (255, 239, 5) BCH are required to be identified, and hence the error correction frame 5 is configured and the frame synchronization thereof is established independent of the synchronization of the transmission frame 17.
Finally, referring to FIG. 10, description will be given of the configuration of the error correction frame 5. In the .[.motion.]. video data 3, corresponding to each block line as the encode unit, there are arranged encoded data for a video frame the first block line 31, the second block line 32, . . . , and the n-th block line 33 located at the lowermost position in the screen, and the unique word F.sub.s 30 identifying the division point of the video frame is added at the top of the data, thereby forming the video frame data 3. Next, subdividing the video frame data 3 from an arbitrary bit position in a unit of 239 bits, a 1-bit error correction frame bit S 34 and an error correction bit ECC 36 of the BCH code are added to the 239-bit unit so as to constitute a 256-bit error correction frame 5. Finally, the 16-phase inter-leave operation is achieved in a unit of .[.16correction.]. .Iadd.16 correction .Iaddend.frames 5 excepting the frame bit S 34, thereby forming the video data 7 to be multiplexed for a generation of the transmission frame 17. In general, since the amount of information of the video frame data 3 is not uniform for each video frame, in order to match the amount of information with the fixed code transmission speed, there is disposed the transmission buffer 2 of FIG. 8 to effect the speed smoothing or matching operation. However, since the capacity of the buffer memory 2 is limited, there may possibly arise an overflow state and an underflow state. In order to prevent such disadvantageous states, according to the CCITT Draft Recommendation H.130 Part .[.B.]. .Iadd.3.Iaddend., there is introduced a control method, namely, the motion video encoder 1 of FIG. 8 stops the encode operation when an overflow occurs, whereas at an occurrence of an underflow state, the pertinent information is forcibly generated by use of, for example, the fixed-length encoding scheme. Furthermore, with consideration of a delay of a response in the control method, the capacity of the buffer memory is increased. The buffer memory occupancy is restricted to be 180 kbits on the transmission side and 220 kbits on the reception side, and the delay associated with the smoothing operation is set to be a relatively large value of about 165 milliseconds (ms).
Since the conventional sub-rate multi-media data transmission control system is configured as described above, there arise the following problems, namely, the transmission frame synchronization must be established independently of the error correction frame synchronization, the size of the apparatus is increased, a long period of time is required to establish all frame synchronizations when the system is applied to the sub-rate ranging from 64 kbps to 384 kbps and hence a countermeasurement is necessary, the capacity of the buffer memory is increased and the configuration of the apparatus is accordingly complicated, and a greater amount of delay occurs in the smoothing operation of the motion video data, which causes a considerable hindrance in case of the sub-rate ranging from 64 kbps to 384 kbps.